Method for Constructing a Floating Unit

ABSTRACT

The present invention relates to a method for constructing a unit adapted to float in a body of water, the method comprising the steps of:
         providing a cavity in the unit such that the cavity is open to the environment surrounding the unit, wherein at least a portion of the cavity is adapted to be located below a still water surface when the unit floats in the body of water, and   determining a first value of at least a first parameter relating to required hydrostatic properties for the unit.       

     The present invention also relates to a unit adapted to float in a body of water. The unit comprises a cavity such that the cavity is open to the environment surrounding the unit. At least a portion of the cavity is adapted to be located below a still water surface when the unit floats in the body of water. Moreover, the present invention relates to a use of a cavity in a unit adapted to float in a body of water.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Patent Application No.61/152,293 which was filed on Feb. 13, 2009 and SE 0900185-0 which wasfiled on Feb. 13, 2009, the entirety of which is incorporated byreference herein.

TECHNICAL FIELD

The present invention relates to a method for constructing a unitadapted to float in a body of water. The method comprises the steps of:

providing a cavity in the unit such that the cavity is open to theenvironment surrounding the unit, at least a portion of the cavity beingadapted to be located below a still water surface when the unit floatsin the body of water, and

determining a first value of at least a first parameter relating torequired hydrostatic properties for the unit.

BACKGROUND

The present invention also relates to a unit adapted to float in a bodyof water. The unit comprises a cavity such that the cavity is open tothe environment surrounding the unit. At least a portion of the cavityis adapted to be located below a still water surface when the unitfloats in the body of water. Moreover, the present invention relates toa use of a cavity in a unit adapted to float in a body of water.

Units adapted to float in a body of water—such as ships or vessels fordrilling and/or production of hydrocarbons—are generally designed tocarry load, i.e. to have a load carrying capacity. As such, in order tocarry the aforesaid load, the unit is designed so as to provideappropriate buoyancy in terms of the magnitude of the buoyancy as wellas the location of the centre of buoyancy. In addition, the buoyancy ofthe unit—when floating in a body of water—should also be designed tocarry the weight of the unit per se, which weight is often referred toas a light unit weight, as well as the weight of inter alia operationalfluids—e.g. ballast water and fuel—required for the unit to functionproperly.

In addition to providing appropriate buoyancy, the unit should also bedesigned to provide appropriate stability characteristics of the unit.Depending on inter alia the vertical centre of gravity of the unit aswell as the magnitude and location of the unit's surface exposed to thewind, the unit should be designed so as to provide a water plane areathe magnitude and location of which provide an appropriately largerighting moment for the unit.

Parameters relating to: the buoyancy, the centre of buoyancy as well asthe magnitude and location of the water plane area of the unit, when theunit is floating in a body of water, may be regarded as parametersrelating to the hydrostatic properties of the unit. As such, whendesigning and constructing a unit adapted to float in a body of water,it is generally of interest to ensure that the actual hydrostaticproperties of the unit meet the hydrostatic properties required for theunit in order to function properly, e.g. in order to be able to providea specific load carrying capacity.

However, during a design and/or construction phase of a unit, it isquite often realized that the initial hydrostatic properties of the unitmay have to be modified in order to meet new requirements for the unit.Traditionally, the need for modified hydrostatic properties is oftenoccasioned by—but is not limited to—the fact that: the magnitude andcentre of gravity of the light unit weight of the constructed unit donot correspond to the values assumed when designing the unit or that themagnitude and/or centre of gravity of the load carrying capacity ischanged during the design and/or construction phase of a unit. It shouldalso be noted that the specified load carrying capacity of a unit maysometimes be changed after the completion of the construction phase of aunit.

In order to modify the hydrostatic properties of a unit during theconstruction thereof, prior art proposes that the unit be furnished withextensions, such as sponsons or fenders, which extensions protrude fromthe original unit and which are adapted to be at least partiallyimmersed in water when the unit is floating in a body of water.Depending on the location of the extensions, at least one parameterrelating to the hydrostatic properties, such as the buoyancy and/orwater plane area, of the unit may be modified such that the modifiedunit presents hydrostatic characteristics which meet the newrequirements for the unit.

However, although the extensions are useful for obtaining desiredhydrostatic properties, the extensions also often introduce otherproblems for the unit. For instance, the extensions generally result inthat the unit will be subjected to increased environmental loads, e.g.loads from waves and currents, and these increased loads may in turnresult in the need for reinforcements of the unit. Moreover, additionalarrangements of the unit, such as propulsion and/or mooring arrangementsmay also have to be modified in order to meet the increasedenvironmental loads emanating from the aforesaid extensions.

Additionally, the unit generally has to be located in a safe location,such as a dock or by a quay, when furnishing the unit with theextensions. Thus, the provision of the extensions of the unit generallyresults in an increased construction time for the unit, which in turnmay result in increased construction costs.

As may be realized from the above, there is a need for improving theprocedure of modifying the hydrostatic properties of a unit.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a method forconstructing a unit adapted to float in a body of water, which methodprovides for that the hydrostatic properties of the unit may be modifiedin a simple manner, even at a late stage of the construction phase ofthe unit.

A second object of the present invention is to provide a method forconstructing a unit adapted to float in a body of water, which methodprovides for that the hydrostatic properties of the unit may be modifiedin such a way that other properties, such as hydrodynamic properties, ofthe unit are not unduly effected.

A third object of the present invention is to provide a method forconstructing a unit adapted to float in a body of water, which methodprovides for that the hydrostatic properties of the unit may be modifiedoutside a dock or a quay.

A fourth object of the present invention is to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

At least one of the above objects is solved by a method for constructinga unit adapted to float in a body of water according to claim 1.

As such, the present invention relates to a method for constructing aunit adapted to float in a body of water. The method comprises the stepsof:

providing a cavity in the unit such that the cavity is open to theenvironment surrounding the unit, at least a portion of the cavity beingadapted to be located below a still water surface when the unit floatsin the body of water;

determining a first value of at least a first parameter relating torequired hydrostatic properties for the unit.

As used herein, the expression “hydrostatic properties” encompasses, butis not limited to, at least one of the following properties of the unit:the buoyancy, the centre of buoyancy, the water plane area and themoment of inertia of the water plane area.

According to the present invention, the method further comprises thestep of fixedly sealing at least a portion of the cavity from thesurrounding environment to thereby form an enclosed volume such that asecond value of the first parameter is obtained, such that the absolutevalue of the difference between the first value and the second value isbelow a predetermined value.

Thus, by providing a method for constructing a unit which methodcomprises the step of modifying a parameter relating to the unit'shydrostatic properties by sealing at least a portion of a cavity, thehydrostatic properties of the unit may be modified at a late stage ofthe construction phase, and even after the completion of theconstruction of the unit in a dock or by a quay, in a simple andstraightforward manner.

Moreover, since the hydrostatic properties of the unit—according to themethod of the present invention—are modified without having to furnishthe unit with protruding extensions, the hydrodynamic properties of theunit are generally only marginally modified, which reduces, and ofteneven removes, the need for reinforcements of the unit and/ormodifications of additional arrangements, such as mooring arrangements,due to the modifications of the hydrostatic properties of the unit.

The expression “modifying the hydrostatic properties of a unit” isintended to mean that a first parameter may be increased, decreased orre-distributed throughout the unit.

According to a preferred embodiment of the present invention, the methodcomprises the step of providing a plurality of cavities in the unit.

By the provision of a plurality of cavities in the unit, the flexibilityas regards how to modify the hydrostatic properties of the unit isenhanced.

According to another embodiment of the present invention, the step offixedly sealing at least a portion of the cavity is performed for atleast two of the plurality of cavities. This further enhances theflexibility regarding how to modify the hydrostatic properties of theunit.

According to a further embodiment of the present invention, the floatingunit comprises a float adapted to be located under the still watersurface, the floating unit further comprises a plurality of supportcolumns, each one of the support columns extending from the float andbeing adapted to intersect the still water surface, wherein the cavityis provided on at least one of the support columns.

According to another embodiment of the present invention, at least onecavity is provided in each one of the support columns.

According to a further embodiment of the present invention, the step ofdetermining the first value comprises a step of determining the weightand/or centre of gravity of the unit.

According to a further embodiment of the present invention, the step ofdetermining the first value comprises a step of determining the buoyancyand/or centre of buoyancy of the unit.

According to another embodiment of the present invention, the step ofdetermining the first value comprises a step of determining a loadcarrying capacity of the unit.

According to a further embodiment of the present invention, the step offixedly sealing at least a portion of the cavity comprises the steps ofproviding the unit with a sealing member, such as a panel, and fixedlyattaching the sealing member to the unit.

A second aspect of the present invention relates to a unit adapted tofloat in a body of water, the unit comprising a cavity such that thecavity is open to the environment surrounding the unit, at least aportion of the cavity being adapted to be located below a still watersurface when the unit floats in the body of water. According to thesecond aspect of the present invention, the cavity is adapted to receivea sealing member to thereby fixedly sealing at least a portion of thecavity from the surrounding environment.

According to a preferred embodiment of the second aspect of the presentinvention, the unit comprises guide means for guiding the sealing memberinto position in the cavity.

A third aspect of the present invention relates to a use of a cavity ina unit adapted to float in a body of water, the cavity being open to theenvironment surrounding the unit, at least a portion of the cavity beingadapted to be located below a still water surface when the unit floatsin the body of water, wherein the use comprises modifying at least onehydrostatic property of the unit by fixedly sealing at least a portionof the cavity from the surrounding environment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means ofnon-limiting examples with reference to the appended figures wherein:

FIG. 1 is a schematic side view of a floating unit according to thesecond aspect of the present invention;

FIG. 2 is a sectional view of a portion of the FIG. 1 unit;

FIG. 3 is a sectional view of a portion of the FIG. 1 unit illustratinga step of the method of the present invention;

FIG. 4 a is a sectional view of a portion of the FIG. 1 unit when asealing member has been attached to the unit;

FIG. 4 b is a top cross section view of a portion of the FIG. 4 a unit;

FIG. 5 is a sectional view of a portion of the FIG. 1 unit illustratinga step of the method of the present invention;

FIG. 6 is a sectional view of a portion of another embodiment of theunit of the present invention;

FIG. 7 is a sectional view of a portion of a further embodiment of theunit of the present invention;

FIG. 8 is a sectional view of a portion of yet another embodiment of theunit of the present invention;

FIG. 9 is a top view of a portion of the FIG. 8 unit;

FIG. 10 illustrates a perspective view of a unit of ring wall type.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be described using examples of embodiments. It shouldhowever be realized that the embodiments are included in order toexplain principles of the invention and not to limit the scope of theinvention, defined by the appended claims.

FIG. 1 illustrates a unit 10 adapted to float in a body of water 12,during the execution of the construction method of the presentinvention. In FIG. 1, the unit 10 is a semi-submersible unit 10 but isshould be noted that the construction method of the present invention isalso applicable for other types of floating units, such as for instanceships or spar buoys (not shown in FIG. 1). Purely by way of example, thebuoyancy of the unit 10 may be above 10 000 metric tonnes and may insome cases even be above 100 000 metric tonnes.

The FIG. 1 semi-submersible unit 10 comprises a float 14, a deckstructure 16 and at least one support column 18 extending from the float14 to the deck structure 16. A support column 18 generally has the shapeof a cylinder the cross-section of which generally is circular orrectangular, although there may of course be other types ofcross-sections. The unit 10 in FIG. 1 has four support columns, whereina first 18 and a second 20 support column are visible. The unit 10 inFIG. 1 has a longitudinal extension indicated by an arrow L and avertical extension indicated by arrow V.

As may be realized from FIG. 1, when the unit 10 is floating in thewater 12 having a still water surface 22, the float 14 is adapted to belocated at least partially beneath the still water surface 22 and thedeck structure 16 is adapted to be located at least partially above thestill water surface 22. In the position illustrated in FIG. 1, the wholeof the float 14 is located beneath the still water surface 22 and thedeck structure 16 is located completely above the still water surface22. Moreover, each one of the columns 18, 20 intersects the still watersurface 22 resulting in a water plane area WPA′, WPA″ for each one ofthe columns which are comprised in the total water plane area WPA of theunit 10.

Moreover, in FIG. 1, the unit's 10 centre of buoyancy CoB is indicated.As may be realized by a person skilled in the art, when the unit 10 isfloating in a body of water 12, the buoyancy B of the unit 10 is definedas the mass of the water displaced by the unit 10. Moreover, the centreof buoyancy CoB is the centre of gravity of the aforesaid displacedwater.

FIG. 1 also illustrates the location of the centre of gravity CoG of theunit 10. Generally, the mass M of the unit 10—which mass M is alsoassociated with the centre of gravity CoG—comprises inter alia the lightunit weight, the weight of operational fluids required for the unit tofunction properly as well as the load carrying capacity of the unit 10.

As may be realized by a person skilled in the art, in order for the unit10 to float at an even keel, there needs to be a balance between themass M and the buoyancy B of the unit 10. For instance, if there is amisalignment between the centre of gravity CoG and the centre ofbuoyancy CoB in a direction parallel to the longitudinal extension L ofthe unit 10, the unit 10 will be subjected to an inclination. Such aninclination is generally undesired, for instance since at least aportion of the deck structure 16 will then be located closer to thestill water surface 22 than in an “even keel” position, which makes thisportion prone to being subjected to wave loading. As previouslydiscussed, such misalignments between the centre of gravity CoG andcentre of buoyancy CoB have previously been corrected by furnishing forinstance the float 14 with one or more sponsons (not shown in FIG. 1),i.e. one or more buoyant extensions protruding from the float 14.

Moreover, if the mass M of the unit exceeds the buoyancy B of the unit10 when the unit is floating at a draught 24 at which the unit 10 isdesigned to float, the unit 10 will decline further into the body ofwater 12 until a balance between the mass M and the buoyancy B isobtained, resulting in that the unit 10 will float at a new draught 24′as indicated by a dotted line in FIG. 1.

Additionally, without going into details, the unit's 10 stability—i.e.capability of withstanding heeling moments—is dependent on inter aliathe centre of gravity CoG, the centre of buoyancy CoB, the magnitude ofthe moment of inertia of the water plane area which in turn is dependenton the water plane area as well as the distances, which distances aremeasured in a plane parallel to the still water plane, between theindividual water plane areas WPA′, WPA″. To this end, it should be notedthat a high vertical centre of gravity generally requires a large momentof inertia of the water plane area. This may for instance be achieved bya large water plane area WPA—i.e. a water plane area having a largemoment of inertia per se—and/or a water plane area WPA constituted by aplurality of individual water plane areas WPA′, WPA″ the distancesbetween which are large.

As such, if it for instance is realized that the vertical centre ofgravity CoG of the unit is higher than what was assumed when designingthe unit 10, the unit 10 may have to be modified in order to compensatefor the aforesaid increase of the vertical centre of gravity CoG. Tothis end, prior art proposes that fenders (not shown in FIG. 1) areattached to at least one of the support columns 18, 20 wherein thefender is buoyant and adapted to intersect the still water surface 22 inorder to provide an additional water plane area WPA′″ to the unit 10.

As may be realized from the above, during its construction, the unit 10may have to be modified for a plurality of different reasons in order toobtain required hydrostatic properties of the unit 10. In order tomodify the unit 10 in a preferred manner, the present invention presentsa method for constructing a unit 10, which method comprises the step ofproviding a cavity in the unit such that the cavity is open to theenvironment surrounding the unit 10. At least a portion of the cavity isadapted to be located below a still water surface 22 when the unit 10floats in the body of water 12.

The unit 10 in FIG. 1 is provided with a plurality of cavities, whereintwo of the cavities 26, 28 are located in the supporting columns 18, 20of the unit 10 whereas two cavities 30, 32 are located in the float 14.Preferred implementations of the cavities 26, 28, 30, 32 will beexplained in detail hereinbelow. However, as a general remark, thecavities are preferably designed so as to prevent fluid passage throughthe unit 10, for example through the support columns 18, 20 or the float14. As such, a component of the unit 10, such as a support column 18, 20or the float 14, provided with a cavity to be used in the method of thepresent invention preferably has a closed circumference throughout thearea of the location of the cavity. In other words, a cavity to be usedin the method of the present invention does preferably not comprise athrough opening in the unit 10.

FIG. 2 illustrates a perspective view of a section of the FIG. 1 unit10, wherein two of the unit's 10 cavities 26, 30, namely a first 26 anda second cavity 30, are shown. As may be gleaned from FIG. 2, the firstcavity 26 is located on the first support column 18 and the first cavity26 is adapted to intersect the still water surface 22, indicated withdotted lines in FIG. 2, when the unit 10 floats in a body of water.Furthermore, FIG. 2 illustrates that the first cavity 26 is located bythe outermost corner of the first support column 18, i.e. the corner 32of the first support column 18 being located at the largest distancefrom the centre of the unit 10. This is a preferred location of thefirst cavity 26 since a subsequent step of sealing at least a portion ofthe first cavity 26 from the surrounding environment will result in asubstantial increase of the stability of the unit 10. However, in otherimplementations of the unit 10, the first cavity 26 may be located inother positions of the first support column 18.

FIG. 2 further illustrates that the first cavity 26 is delimited by aplurality of panels, namely a bottom panel 34, a top panel 36 and afirst and a second side panel 38, 40. As may be realized when studyingFIG. 2, of the panels delimiting the first cavity 26, the bottom panel34 is located closest to the float 14 in the vertical direction Vwhereas the top panel 36 is located farthest away from the float 14 inthe vertical direction V. Moreover, the first and second side panels 38,40 extend from the bottom panel 34 to the top panel 36. The panels arepreferably steel plates and the panels are preferably attached to oneanother by means of tight joints, such as weld joints.

Purely by way of example, the volume of the first cavity 26 may bewithin the range of 0.02-0.001, preferably 0.01-0.004, of the totalvolume displaced by the unit 10 when the unit 10 floats at anoperational draught. Moreover, again purely by way of example, thehorizontal cross sectional area of the first cavity may be in the rangeof 0.1-0.005, preferably 0.07-0.01, of the total water plane area of theunit 10.

In a similar manner as the first cavity 26, the second cavity 30 isdelimited by a plurality of panels forming a notch in the float 14. Thesecond cavity 30 is preferably located on the outside of the float 14such that a subsequent step of sealing at least a portion of the secondcavity 30 from the surrounding environment generally will result in asubstantial change of the centre of buoyancy CoB of the unit 10. Purelyby way of example, the volume of the second cavity 30 may be within therange of 0.1-0.001, preferably 0.01-0.004, of the total volume displacedby the unit 10 when the unit 10 floats at an operational draught.

The method of the present invention also comprises a step of determininga first value of at least a first parameter relating to requiredhydrostatic properties for the unit 10.

As has been discussed hereinabove, the hydrostatic properties may berelated to a plurality of properties. As such, the step of determiningthe aforesaid first value, may in some embodiments of the method of thepresent invention comprise a step of determining the mass M and/orcentre of gravity CoG of the unit 10. This step may be performed byactually weighing the unit 10, by procedures known by a person skilledin the art, or by assembling information as regards the mass and centreof gravity for components—which components are considered relevant froma weight point of view—forming a part of the unit 10.

In addition to, or instead of, the step presented above, in embodimentsof the method of the present invention, the step of determining thefirst value may comprise a step of determining a load carrying capacityof the unit 10.

It should be noted that none of the steps above, be it a step ofdetermining the mass or the load carrying capacity of the unit, needs tobe performed when the unit is in a dock or by a quay. Instead, the stepsas presented hereinabove may be performed after the unit 10 has left theconstruction site and in some embodiments of the method of the presentinvention, the step of determining the first value of a first parametermay actually be performed when the unit is in its operating location,e.g. travelling at sea or being moored to a specific operating location,and even when the unit 10 is in an operating condition.

Irrespective of when the step of determining the first value isperformed, the first value is preferably compared to the actual value ofthe first parameter relating to the hydrostatic properties of theconstructed unit 10, i.e. the unit including the cavities 26, 30. If it,from the aforesaid comparison, is realized there is a misalignmentbetween the first value and the actual value of the first parameter, theunit 10 may have to be modified in order to correct the aforesaidmisalignment.

The method steps above are exemplified hereinbelow by means ofnon-limiting examples.

In a first example, the first parameter relating to required hydrostaticproperties is the buoyancy B₁ of the unit 10. As such, the step ofdetermining a first value of the parameter may comprise a step ofdetermining the mass M of the unit 10 and the first value B₁ should thuspreferably correspond to the mass M. If the mass M is larger than theactual buoyancy B_(A) of the unit 10, when the cavities are open, atleast a portion of at least one of the cavities is sealed such that asecond value of the buoyancy B₂ is obtained and such that the difference|B₂-B₁| between the first and second values are below a predeterminedvalue, which predetermined value may be regarded as a tolerance for themethod.

In a second example, the first parameter is the horizontal centre ofbuoyancy HCB₁ of the unit 10. As such, the step of determining a firstvalue of the parameter may comprise a step of determining the horizontalcentre of gravity HCG of the unit 10 and the first value HCB₁ shouldgenerally substantially correspond to the horizontal centre of gravityHCG. If the horizontal centre of gravity HCG differs from the actualhorizontal centre of buoyancy HCB_(A) of the unit 10, when the cavitiesare open, at least a portion of at least one of the cavities is sealedsuch that a second value of the horizontal centre of buoyancy HCB₂ isobtained and such that the difference |HCB₂—HCB₁| between the first andsecond values is below a predetermined value.

It should be noted that in the second example above, the horizontalcentre of buoyancy HCB is generally constituted by two components, alongitudinal centre of buoyancy LCB and a transversal centre of buoyancyTCB. Correspondingly, the horizontal centre of gravity HCG is generallyalso constituted by two components, a longitudinal centre of gravity LCGand a transversal centre of gravity TCG. However, in someimplementations of the second example, the first parameter may be chosenso as to only relate to only one of the aforesaid components, e.g.either the longitudinal centre of buoyancy LCB or the transversal centreof buoyancy TCB which thus should be compared to the correspondingcomponent of the horizontal centre of gravity.

In a third example, the first parameter relating to required hydrostaticproperties of the hull is water plane area WPA₁—for instance bothmagnitude and location of the water plane area—of the unit 10. As such,the step of determining a first value of the parameter may comprise astep of determining the vertical centre of gravity VCG of the unit 10and from that information determining the required magnitude andposition of the water plane area WPA₁ in order to obtain a unit 10 withsufficient stability characteristics. If the water plane area WPA₁differs from the actual water plane area WPA_(A) of the unit 10, whenthe cavities are fully open, at least a portion of at least one of thecavities is sealed such that a second value of the water plane area WPA₂is obtained and such that the difference |WPA₂-WPA₁| between the firstand second values are below a predetermined value.

It should be noted that the first parameter relating to requiredhydrostatic properties of the unit may in some embodiments of thepresent invention be determined by combining some or all of theparameters from the above examples.

As previously mentioned, the aforesaid predetermined value may beregarded as a tolerance for the method. The magnitude of thepredetermined value may be selected from case to case based on interalia the design of the unit as well as the hydrostatic propertyconcerned. Purely by way of example, the predetermined value may beselected as a percentage of the first value of the first parameter. Assuch, again purely by way of example, the predetermined value may be setto be 10%, preferably 5%, more preferably 1% of the first value.

The method of the present invention further comprises a step of fixedlysealing at least a portion of the cavity 26, 30 from the surroundingenvironment to thereby form an enclosed volume such that a second valueof the first parameter is obtained, such that the absolute value of thedifference between the first value and the second value is below apredetermined value. Examples of how this is done are presented in FIG.3.

FIG. 3 illustrates how a first panel 42 is inserted into the firstcavity 26. The first panel 42 preferably is a metal plate of a metalwhich is similar or the same as the metal of the column 18 and/or thepanels 34, 36, 38, 40 partially delimiting the first cavity 26. Thefirst panel 42 may be inserted into the first cavity 26 by using alifting arrangement such as a crane (not shown). Optionally, the firstpanel 42 may be inserted into the first cavity 26 when the unit 10 isfloating in a body of water at an appropriate draught such that thefirst panel 42 may be floated into position, for instance using a barge(not shown). In order to facilitate the insertion of the first panel 42into the first cavity 26, the first column 18 preferably comprises guidemeans (not shown), such as outwardly extending pins, for guiding thepanel into the first cavity 26. In addition, the first panel 42 may beprovided with auxiliary guide means (not shown), such as openings,adapted to interact with the guide means of the first column. FIG. 3also illustrates that the unit 10 is provided with a second panel 44 inorder to seal the second cavity 30.

The first 42 and second 44 panels may be fixedly attached to the unit bymeans of one or more joints. Purely by way of example, such a joint maybe a weld joint and/or a bolt joint. When attaching a panel 42, 44 to aportion of the unit 10 being located at least partially beneath thestill water level, a habitat may be used in order to provide asubstantially dry environment for the attachment operation.

FIG. 4 b illustrates a cross section from above of the first column 18when the first panel 42 has been inserted into the first cavity 26.Moreover, the first panel 42 has been attached to the first column 18 bymeans of tight joints. As may be gleaned from FIG. 4 b, once the firstpanel 42 is attached to the first column 18, an enclosed volume 45 isformed in the first cavity 26. The enclosed volume 45 will be buoyantwhen at least partially submerged into a body of water, thus theenclosed volume 45 increases the buoyancy, as well as the water planearea WPA, of the unit 10. Moreover, as may be realized from FIG. 4 b, aportion 46 of the cavity 26 is still open to the environment surroundingthe unit 10. The open portion 46 may be regarded as additional buoyancyand/or water plane area reserve, which may be fixedly sealed at a laterstage of the life of the unit 10, should increased buoyancy and/or waterplane area be subsequently required.

FIG. 5 illustrates an optional procedure of fixedly sealing at least aportion of a cavity from the surrounding environment. In FIG. 5, thesecond cavity 30 is used as an example although it should be realizedthat the procedure is equally applicable for any one of the cavities ofthe unit 10. As may be gleaned from FIG. 5, rather than closing theentire second cavity 30 by a panel, only a portion—in this case in thelongitudinal direction L—of the second cavity 30 is fixedly sealed fromthe surrounding environment. To this end, in the embodiment of themethod of present invention illustrated in FIG. 5, the method comprisesa step of providing a coffer 48—or end piece—which is inserted into thesecond cavity 30 and subsequently fixedly attached to the float 14.Preferably, the coffer 48 is buoyant such that it may be floated intothe second cavity 30.

Instead of, or in addition to, the provision of the coffer as discussedwith reference to FIG. 5 hereinabove, in order to be able to close onlya portion of a cavity in its longitudinal extension—which extension maycoincide with the longitudinal extension L of the unit 10 as is the casefor the second cavity 30 in FIG. 5—the cavity may be divided into aplurality of compartments. An example of a cavity provided with aplurality of compartments is presented in FIG. 6, wherein the secondcavity 30 contains a first 50 and a second 52 compartment. As such, inthe implementation illustrated in FIG. 6, the second cavity 30 has beenpartitioned by an additional panel 54 or partition wall. Thus, eitherone, or both, of the first 50 and second 52 compartments may later on befixedly sealed by corresponding sealing members. In FIG. 6, the sealingmembers are exemplified by two sealing panels 56, 58, but the sealingmembers may in other embodiments of the method of the present inventionbe coffers (not shown in FIG. 6) similar to the one illustrated in FIG.5 for instance.

To this end, FIG. 7 illustrates another implementation of a sealingmeans which may be used for sealing the first compartment 50. The FIG. 7sealing means is a coffer 60 the depth d of which may be smaller thanthe depth D of the first compartment 50. Moreover, the FIG. 7 coffer 60comprises an outer flange 62 provided with openings 64 for fasteningmeans such as bolts (not shown in FIG. 7). The FIG. 7 coffer 60 willthus act as a plug when inserted in the first compartment 50 andattached to the unit by means of e.g. a bolt joint. One advantage of theFIG. 7 coffer 60 is that it may be used for sealing the firstcompartment 50 in a straightforward manner, even if the firstcompartment is located below the still water surface. A method ofsealing the first compartment 50 by means of the coffer 60 may comprisethe steps of: filling the coffer 60 with water such that it submerges,guiding the coffer into the first compartment 50, attaching the cofferto the unit 10 and removing water from the first compartment 50 (andpossibly also from the coffer 60).

FIG. 8 and FIG. 9 illustrate an optional implementation of the cavity26. As may be realized from FIG. 8, instead of providing a cavity theopening of which is substantially of the same size as the cavity itself,the FIG. 8 cavity 26 is open to the surrounding environment by means ofa plurality of openings 66, 68 in a panel 70 outwardly delimiting thecavity 26. In the implementation illustrated in FIG. 8 and FIG. 9, thepanel 70 has two openings. As such, if the two openings 66, 68 would nothave been present in the panel 70, the panel 70 would have formed a partof the outer skin of the unit. The openings 66, 68 are sufficientlylarge so as to allow a free sea water flow in and out of the cavity 26.If the additional buoyancy and/or water plane area of the cavity 26 isrequired, the cavity 26 is sealed from the surrounding environment byfixedly sealing the openings 66, 68, for instance by using sealingmembers such as sealing panels (not shown).

Moreover, it should be noted that the step of fixedly sealing the atleast a portion of the cavity from the surrounding environment maycomprise a step of at least partly emptying the portion of the cavityfrom sea water. Such a step may for instance be performed after asealing member has been attached to the unit 10 for sealing the portionof the cavity. The step of emptying the portion of the cavity from seawater may typically be performed for a cavity at least a portion ofwhich is located beneath the still water surface 22 during theattachment of the sealing member to the unit.

As regards the embodiments of the method present invention as presentedhereinabove, it should be noted that although a semi-submersible unit 10comprising a plurality of columns 18, 20 has been used as an example ofa unit 10 of the present invention, the method as claimed in claim 1 isalso applicable for other types of units 10. As an example, FIG. 10illustrates a unit 10 of a so-called ring wall type having an inner 60and an outer wall 62 forming a closed hollow wall structure 64. The FIG.7 unit 10 is provided with a first 26 and a second 30 cavity, whereinthe first cavity 26 is adapted to intersect a still water surface whenthe unit 10 is floating in a body of water whereas the second cavity 30is adapted to be located below the still water surface. As such, itshould be realized that the present invention is not limited to theembodiments described hereinabove and illustrated in the drawings.Rather, a person skilled in the art will realize that many changes andmodifications may be performed within the scope of the appended claims.

1. A method for constructing a unit adapted to float in a body of water,said method comprising the steps of: providing a cavity in said unitsuch that said cavity is open to the environment surrounding said unit,at least a portion of said cavity being adapted to be located below astill water surface when said unit floats in said body of water;determining a first value of at least a first parameter relating torequired hydrostatic properties for said unit; and fixedly sealing atleast a portion of said cavity from the surrounding environment tothereby form an enclosed volume such that a second value of said firstparameter is obtained, such that the absolute value of the differencebetween said first value and said second value is below a predeterminedvalue.
 2. The method according to claim 1, wherein the method comprisesthe step of providing a plurality of cavities in said unit.
 3. Themethod according to claim 2, wherein said step of fixedly sealing atleast a portion of said cavity is performed for at least two of saidplurality of cavities.
 4. The method according to claim 1, wherein saidunit comprises a float adapted to be located under said still watersurface, said floating unit further comprises a plurality of supportcolumns, each one of said support columns extending from said float andbeing adapted to intersect said still water surface, wherein said cavityis provided on at least one of said support columns.
 5. The methodaccording to claim 4, wherein at least one cavity is provided in eachone of said support columns.
 6. The method according to claim 1, whereinsaid step of determining said first value comprises a step ofdetermining at least one of a weight and centre of gravity of said unit.7. The method according to claim 1, wherein said step of determiningsaid first value comprises a step of determining at least one ofbuoyancy and centre of buoyancy of said unit.
 8. The method according toclaim 1, wherein said step of determining said first value comprises astep of determining a load carrying capacity of said unit.
 9. The methodaccording to claim 1, wherein said step of fixedly sealing at least aportion of said cavity comprises the steps of providing said unit with asealing member, such as a panel, and fixedly attaching said sealingmember to said unit.
 10. The method according to claim 1, furthercomprising modifying at least one hydrostatic property of said unit byfixedly sealing at least a portion of said cavity from the surroundingenvironment.
 11. A unit adapted to float in a body of water, said unitcomprising a cavity such that said cavity is open to the environmentsurrounding said unit, wherein at least a portion of said cavity isadapted to be located below a still water surface when said unit floatsin said body of water, wherein said cavity is adapted to receive atleast one sealing member to thereby fixedly seal at least a portion ofsaid cavity from the surrounding environment.
 12. The unit according toclaim 11, wherein said unit comprises guide means for guiding saidsealing member into position in said cavity.
 13. The unit according toclaim 11, further comprising a plurality of cavities in said unit. 14.The unit according to claim 13, wherein at least a portion of at leasttwo of said plurality of cavities are fixedly sealed from thesurrounding environment.
 15. The unit according to claim 11, furthercomprising a float located under said still water surface, said floatingunit further comprising a plurality of support columns, each one of saidsupport columns extending from said float and adapted to intersect saidstill water surface, wherein said cavity is provided on at least one ofsaid support columns.
 16. The unit according to claim 15, wherein atleast one cavity is provided in each one of said support columns.